Long story, short suggestion:
If the root folder that's being monitored is unavailable, MediaMonkey probably shouldn't prune the contents of that folder and its subfolders from the library even when 'remove unavailable items' is ticked - or should at least ask the user. Obviously, if subfolders (only) are missing their contents should be pruned. This means that in the case of hard drive or network failure the library isn't lost while keeping the value of 'remove unavailable items'. It also shouldn't require much programming to achieve

I'm not sure if you're understanding? If I'm monitoring the "My Music" folder and "My Music" is unavailable, I suggest items shouldn't be removed without prompting the user. Obviously if anything within what I've asked to be monitored is missing (a whole genre's folder or artist's folder for example), it should be got rid of.
I don't know specifically about drives, but I do know about network locations - and lost the whole library as a result. It's my fault, I'm not whining but I can't imagine people would often have set up a root folder to monitor, that they delete or rename or is inaccessible, but they want MM to keep monitoring the folder that doesn't exist...
Maybe other people use the file monitor very differently from me, but I only have a very few root folders selected in it.

And why would the folder be temporarily unavailable? Drives can be (think external/network), but folders don't tend to be temporarily unavailable. Thus it's logical that the root folder gets the same treatment as sub-folders.

Well I am talking about a network location - network connection down, folder temporarily unavailable. I'm suggesting the important thing is not the drive but the root folder.

I guess my question is why not? Why not at least ask? I'd have thought it would be rare that it would be triggered and rare users would want the answer to be yes.
For me, the purpose of the feature is to tidy up everything else, not remove my entire collection.

Like I said if the drive (internal/external or network location) is unavailable the files on the drive shouldn't be removed from the Library. If files are removed from an unavailable drive that would be a bug.

How are you connecting to the network drive (IP, UNC or mapped drive)?

I submitted ticket YAW-575064. Apparently, it is expected behaviour.
So, I repeat my wish that if the root folder or network location or drive - whichever you'd prefer - is unavailable, MM prompts the user to ask if all items should be removed.
Personally, I don't see that behaviour as the point of the 'remove unavailable items' option, but it's clearly the consequence. I see the point being that it keeps your library in sync with a collection that you edit from outside of MM. It also makes the option very dangerous on a network location, as I discovered.
Do I have support from you Lowlander?

Okay, for an experiment I put some files on an external drive. When I unplug the external drive, the files don't disappear from the library, so you were right. However, for the network location they disappear, at least with UNC.
So it sounds like it is a bug, because the expected behaviour is that if the root location is unavailable it doesn't remove the files from the library. I'll try reopening the ticket and see what happens.
You can do the experiment too. Add a network folder to the file monitor, turn on 'remove unavailable items', switch off networking, see what happens